His point is undeniable.
When the city finally fell to a Germanic horde, Gibbon wrote that the gate was opened by someone who had more sympathy with the forces outside, than with the people inside.
Whether that detail is accurate, the Western Empire was being overrun by migrant forces for many decades.
There is no question the Empire fell to this gradual takeover, and not to its foreign enemies to the East and South.
Look how the the UK’s largest city has already fallen to one of the STABBY HEADCUTTER INVADERS.
There was a shift in the composition of the Empire’s army as well. Germanic warriors were making up the bulk of the auxiliary forces. The empire relied on the loyalty of the Germanic commanders because they had no other choice.